Crystalline lamellae layers

The long period, spacing between adjacent crystalline lamellae layers, can be estimated from a circular averaged 1-D SAXS data. The lamellar morphology of iPP in the blends was also determined from the position of the SAXS maxima. 

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.

The development of order in each layer is actually the growth of crystalline lamellae. We show in Fig. 22 a typical snapshot of the system of 640 chains of Cioo (64 000 atoms) at 12.8 ns after quenching from 600 K down to 350 K. It is clearly noticed that the stacked lamellae grow from both side substrates. For further analysis of the data we need to extract crystalline regions. In order to... 

Homogeneous melt, Todt < Tc > Tg. In diblock copolymers exhibiting homogeneous melts, microphase separation is driven by crystallization if Tg of the amorphous block is lower than Tc of the crystallizable block. This generally results in a lamellar morphology where crystalline lamellae are sandwiched by the amorphous block layers and spherulite formation can be observed depending on the composition [6-10]. 

Fig. 6 Micromechanical model of a section of a semi-crystalline polymer with lamellae oriented perpendicular to the principal stress direction showing the long period L and the thicknesses of crystalline (Lc) and amorphous layers (La) the latter are composed of loose segments, entangled chains and more or less extended tie molecules. Large forces can be transferred at those points (o) where highly extended tie molecules (eTM) enter crystalline lamellae...

An experimental relationship between the microhardness and elastic modulus (E) has been found for various PE materials with different crystallinity values (Flores et al.., 2000). It is important to realize that microhardness - the plastic deformation of crystals at high strains - primarily depends on the average thickness and perfection of the nanocrystals, whereas in the case of the modulus, the elastic response at low strains is dictated by the cooperative effects of both microphases, the crystalline lamellae and the amorphous layer reinforced by tie molecules. The... 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

The kinetics of the OSO4 treatment as revealed by microhardness measurements has been examined. The increase in microhardness has been explained in terms of the large reduction in molecular mobility of the amorphous, interlamellar layers, Chlorosulphonation produces an initial microhardening of the amorphous phase, in which reaction time, temperature and the molecular weight play an important role. The OSO4 reaction induces an additional microhardening at the surface of crystalline lamellae. Results reveal that Ha increases, after the above chemical reactions, from nearly zero up to values of 300 MPa (Baltd Calleja et ai, 1997). 

Figure 3.12 shows an important characteristic of the multi-layer stack, i.e. the repeat period 4, which consists of the combined thickness of one crystalline lamella and one non-crystalline region between lamellae and is usually of the order of tens of nanometers. Other important questions about the stacks are the following, (a) What are the individual thicknesses of the crystalline and non-crystalline layers (b) What is the nature of the non-crystalline material (c) Can the crystalline and non-crystalline layers... 

Three growth regimes occur in polymer crystals. Regime 1 completes a surface layer of crystalline lamellae before starting another layer. Regime 3 starts new layers before the underlying layers are complete. 

Whatever the morphology of a semi-crystalline polymer such as HDPE, the irreducible structural unit involved in the plastic flow is a pair of coupled crystalline lamellae and its complement of an amorphous layer in between, where the ratio of the thickness of the crystalline lamellae to that of the pair of crystalline and amorphous layers is, to a first approximation, equal to the crystallinity of the polymer. 

The semi-crystalline HDPE being modeled is initially of a sphernlitic morphology described in Chapter 2. It is made up of a 3D packing of crystalline lamellae and their attached amorphous layers as idealized in Fig. 9.25(a). The basic elements of the spherulite are two-phase composite inclusions that consist of integrally coupled crystalline lamellae and their associated amorphous layers between lamellae. Owing to their large aspect ratio, the composite inclusions are modeled as infinitely extended sandwiches with a planar crystalline/amorphous interface as shown in Fig. 9.25(b). Each composite inclusion I is characterized by its interface normal rt and the relative fractional thicknesses and / = 1 / of... 

Fig. 13.30 A TEM micrograph of Nylon-6 modified with EPDR rubber in which rubber particles are surrounded by preferentially ordered parallel crystalline lamellae forming material layers of low plastic resistance. The micrograph was stained with phosphotungstic add, which makes crystalline lamellae stand out over a stained amorphous background (from Muratoglu et al. (1995b) courtesy of Elsevier).

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